Electrotinning process to prevent plating on the cathode contact roll

ABSTRACT

IN AN ELECTROTIMING PROCESS, A METALLIC SHIELD ISPOSITIONED NEAR THE CATHODE CONTACT ROLL AND MAINTAINED AT A SLIGHTLY MORE NEGATIVE (MORE CATHODIC) POTENTIAL THAN THE CONTACT ROLL SO AS TO PREVENT PLATING ON THE CONTACT   ROLL. SPECIFICALLY, THE METALLIC SHIELD IS OPERATED NEAR THE CATHODE ROLL IN A CONTINUOUS WIRE OR STRIP PLATING UNIT.

April 24, 1913 O. B. MATHRE ET AL ELECTROTINNING PROCESS TO PREVENT PLATING ON THE CATHODE CONTACT ROLL Original Filed Jan. 15, 1968 INVENTORS OWEN BERTWELL HATHRE DONALD ARTHUR SWALH'EII ATTORNEY United States Patent this application Feb. 11, 1971, Ser. No. 114,664

Int. Cl. C23b /14, 5/58, 5/68 US. Cl. 20428 3 Claims ABSTRACT OF THE DISCLOSURE In an electrotinning process, a metallic shield is positioned near the cathode contact roll and maintained at a slightly more negative (more cathodic) potential than the contact roll so as to prevent plating on the contact roll. Specifically, the metallic shield is operated near the cathode roll in a continuous wire or strlp plating unit.

RELATIONSHIP TO OTHER APPLICATIONS This application is a division of our copending US. application Ser. No. 703,830, filed Jan. 15, 1968, now abandoned, which is a continuation-in-part of our US. application Ser. No. 685,452, filed Nov. 24, 1967, now abandoned.

BACKGROUND OF INVENTION A horizontal-type plating line is used commercially for electrotinning steel strip using the Halogen Tin Process. The first deck of the plating unit consists of a series of plating cells with a similar series of cells located in an overhead position and constituting a second deck. The steel strip is held in a horizontal position and is guided through the unit by pairs of rolls located between each of the plating cells. The bottom side of the strip is plated in the first deck. The steel strip then passes vertically at the exit end of the first deck, reverses direction in the second deck in which the other side is plated. In operation, as the strip travels through the cell, particularly at speeds in the range of 1000 to 2000 ft./min., it pumps electrolyte toward the exit end and the electrolyte piles up against the face of the contact roll. Since the anodes of the cell extend beyond the edges of the strip, it is readily apparent that the current from the ends of the anodes can also fiow directly to the surfaces of the contact roll extending beyond the edges of the strip and plate tin on the ends of the roll. Very little, if any, tin deposits on the surface of the contact roll directly in contact with the strip.

The problem of plating on the contact roll is not serious when a new steel or chromium-plated roll is installed in a commercial unit. The tin is removed by periodically standing with an appropriate abrasive. However, it is impractical to selectively remove tin without removing some metal from the surface of the roll. 'In a relatively short time the surface of the roll is not a true cylinder. When this condition develops, good contact is not maintained between the nip of the contact roll and the strip. This leads to an increase in the voltage drop in areas of poor contact and excessive rate of plating on the roll. The rolls are removed from the unit at periodic intervals and are ground and polished to a cylindrical form. After several such cycles the roll is discarded, and a new roll is installed in the unit. It is obvious that the cost of sanding the rolls during operation, labor time in changing rolls, loss of operating time, and expense of new rolls represent costly maintenance items.

3,729,390 Patented Apr. 24, 1973 "ice Numerous attempts to block the direct electrical path between the anode and the cathode contact roll, and thereby prevent plating on the rolls in the commercial units, have been unsuccessful. It is impractical to completely block the direct electrical path with a permanently fixed insulating shield because the width of the strip being plated is variable.

SUMMARY OF INVENTION According to the present invention there is provided a process for the continuous electrotinning of a wire or strip substrate from an electroplating bath to provide a tin coating thereon, the improvement comprising: immersing a metallic shield into the bath near and essentially parallel to the axis of the cathode contact roll and between the anode and cathode contact roll and maintaining the shield at a potential about 0.030.2 volt more negative than the surface of the cathode contact roll.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view of a commercial plating cell with the metallic shield in place;

FIG. 2 is a plan view of a commercial plating cell with the metallic shield in place;

FIG. 3 is a plan view of a laboratory plating cell with the metallic shield; and

FIG. 4 is a side sectional view of a laboratory plating cell with the metallic shield.

DETAILED DESCRIPTION OF THE INVENTION The apparatus and process of the present invention briefly comprise the positioning and operating of a metallic shield near the face of the cathode roll and maintaining it at a negative potential with respect to the cathode roll. Since the negative potential on the shield is maintained and controlled at a slightly more negative (more cathodic) value than the roll, tin plates preferentially on the shield and plating on the roll is thereby prevented.

The structure and operation of a commercial plating cell are shown in FIGS. 1 and 2. A plating cell 10 is located under the metal strip 11 to be plated and the anodes 12 are generally positioned at a distance of about 1.25 to 1.5 inches from the strip. The metal strip 11, usually steel, is held in a horizontal position by the pairs of rolls 13, 14 and 15, 16. Electrolyte 17 is pumped into the cell and maintained at a level slightly above strip 11 and overflow of electrolyte from the cell returns to a storage tank (not shown) through downcomers 18 and 19. Conductor or cathode rolls 13 and 15 are connected to the negative terminal of a direct current source (not shown) by the cathode bus bars (not shown) while the positive terminal of the direct current source is connected to conducting anode supports 20. Moderately high pressure is applied between the rubber-covered back-up rolls 14 and 16 and the cathode rolls 13 and 15 in order to make good electrical contact between the cathode rolls and the strip so as to minimize the voltage drop.

The metallic shield 21 is shown in one piece and extends across the width of the cell 10 parallel to cathode roll 15. Although from a practical standpoint a one-piece conductor shield may be easier to install and operate, the shield can extend from each side of the plating cell wall to approximately the position of the edge of the wire or strip plated. In view of the variations in width of strips plated, one shield on each side will sufiice if it extends to a position above the strip corresponding to the minimum width of strip plated.

Although the connection of the shield to the cathode bus bar is not shown in FIGS. 1 and 2, the shield may be connected to the cathode bus bar using a suitable resistance or other method to insure that the shield is at a negative potential suflicient to plate on the shield and prevent plating on the surface of the roll. The cathode bus bar is normally at a slightly more negative potential than the surface of the roll because of the voltage drop in the commutator or other systems used to electrically connect the bus bar to the revolving roll.

While the negative potential of the shield only must be such as to cause plating on the shield and prevent plating on the surface of the cathode contact roll, it is preferred that the shield be at least about 0.03 volt more negative than the surface of the roll. For a Halogen Tin plating bath such as described in Schweikher US. Pat. 2,407,579, it has been found that a negative potential within the range of about 0.03-0.20 volt more negative than the surface of the roll is sufficient. There are a number of factors involved in plating which will determine what negative potential is sufficient to plate on the shield and prevent plating on the cathode roll. Knowing these factors, it is within the skill of the art to determine the operable shield negative potential difference for a particular plating system. Some of these factors include the spacing between the shield and face of the roll, the degree of agitation of the electrolyte between shield and face of the roll, variation in composition of the eelctrolyte and the types of electrolytes used.

The distance of the shield from the surface of the roll 15 is not critical; however, positioning the shield as close to the surface of the roll as feasible without interference with mechanical rotation of the roll is desirable in order to minimize the amount of tin plating on the shield. The vertical position of the lower edge of the shield with respect to the strip also is not critical but a distance of 1 to 2 inches from the strip, generally, is suflicient. Although the curvature of the shield is similar to the contour of the roll, the shape is not a critical factor since a flat surface screen properly positioned in the cell will also completely eliminate plating on the roll.

It has been discovered that tin does not deposit on the surface of the conductor roll near the nip where it makes contact with the strip, but that the tin actually plates on the face of the conductor roll, generally, one to two inches from the nip position. This portion of the roll surface including the periphery to the top surface of the electrolyte pile-up in front of the roll is closer to the end of the anode bed than the surface of the roll near the nip. The resistance to flow of current between the anode bed and the front surface of the conductor roll is lower and, consequently, tin plates preferentially on this surface. Numerous attempts at installing insulating-type shields between the roll surface and the anodes merely shifted the position of plating on the roll but did not eliminate it.

The laboratory plating cell shown in FIGS. 3 and 4 was used to test the effect of the distance of the shield to the roll on plating.- Basically, the geometric configuration was similar to a section of the commercial plating cell and consisted of a portion of the anode bed, a band of strip and the end of a plating roll. The anode 22, strip 23, and a section of a simulated plating roll 24 are mounted in a vertical position in cell 25. The length of the anode 22 was approximately 10.5 inches. Since plating on the roll is only influenced by the anode surface closest to the roll, it was not deemed necessary to install a 60- inch commercial length anode in the laboratory plating unit. The distance between the anode 22 and the strip 23 was maintained at 1.25 inches. The distance from the end of the anode compartment 26 to the nip between the roll and the strip 27 was 16.25 inches. Basically. the simulated roll 24 consisted of a sheet of steel shaped to conform to the peripheral face of a -inch diameter roll. The back face or surface of the simulated roll was masked with tape to restrict plating to the convex surface of the roll.

A side view of the plating cell is shown in FIG. 4. The strip of steel 23 shown by the narrow band at the bottom of the cell was approximately 1" x 27" in length. The anode 22 extended about 5 inches above the top edge of the strip of steel. The cathode or simulated roll 24 also extended 5 inches above the top of the strip. The strip of steel 27 was attached to 23 and 24 by means of several small screws to insure good electrical contact of the strip. Solution circulation tubings 28 and 29 are also shown in this view. The solution discharge from tubing 28 circulated electrolyte at a fast rate between the anode and the strip, and solution discharge from tubing 29 was directed toward the face of the conductor roll.

A 4-mesh steel screen 31 with a wire diameter of 0.047 inch was used for most of the tests and had a shape similar to the peripheral contour of the plating roll 24. The peripheral dimension of the simulated roll 24 from the nip 27 to the side wall of the cell was 4 inches. The peripheral dimension of the shield from the side wall of the cell to the approximate position shown in FIG. 3 was 1.5 inches with the edge of the screen about 0.75 inch from the strip. The level of electrolyte 30 in the cell was maintained at a depth of 6 inches from the top of the strip. A series of tests were made varying the vertical distance of the shield from the top edge of the strip 23 and the distance of the shield from the surface of the roll. The screen 3 1 was connected electrically or shortcircuited to the steel strip 27 with a very low resistance copper wire conductor. The negative terminal from the rectifier was also connected directly to the strip 27. The current was maintained constant at 15 amperes for a plating time of 10 minutes for each of the plating tests. After plating, the simulated roll and strip assembly was removed and examined for deposition of tin on different areas. The results of the tests are given in Table I.

Plating on strip distance beyond anode, inches Vertical distance from strip, inches Plating on simulated roll Distance of shield to roll, inches 0 9. 5 None.

Do. x area. None.

Do. MI! X 1% area.

1 Corresponds to area on roll 24 at lower left corner opposite the strip (see Figure 4).

The results given in Table I show that the position with respect to distance from the roll surface and distance from the edge of the strip is not critical.

In the tests described herein, the shield was connected directly to the strip 27 supporting the roll 24. Drop in voltage in the commutator used to transfer current from the cathode bus bar to the roll in the commercial units may be higher than 0.075 volt, particularly if the brushes are badly worn. If the drop in voltage between the cathode bus bar and surface of the roll is abnormally high, the shield will be at a more negative potential than required to prevent plating on the roll. Under these conditions, the amount of current flowing to the shield can be reduced and readily controlled by installing a resistance in the circuit from the shield to the cathode bus bar.

In the tests described herein, a steel screen was used as a material of construction for the shield. A perforated shield or screen-type material is preferred because solution can readily pass through the shield and the fluid flow characteristics within the cell are not changed appreciably. This is important, particularly at high strip speeds (1500-2000 ft./min.) because the electrolyte near the ends of each cathode roll at the exit end of the cell reverses direction of flow and returns toward the entry end of the cell.

Other modifications include expanded metal, grids and other types of construction which allow free flow and circulation of electrolyte such as metal rods, wires or stranded cables which can be continuously drawn through and from the bath for cleaning and reintroduction. Solid types of construction for the shield are also satisfactory for applications where free-flow of electrolyte is not an important factor such as low-speed wire or strip lines.

Further modifications of the invention include operating with a shield fabricated from other conducting metals such as tin, stainless steel, chromium-plated steel, copper, brass and other materials in addition to steel. Since it is desirable to recover the plated metal from the shield at periodic intervals and re-use the shield, the choice of preferred metal for construction of the shield may depend on economics and ease of removal of plated metal from the surface of the shield.

The embodiment of the invention as described herein covers the principle of a metallic shield to prevent plating on the cathode rolls in the horizontal-type Halogen Tin units used for continuous electrotinning of strip steel. However, the invention is not limited to this electrotinning process. Plating of cathode rolls and problems associated therewith are also serious in other types of equipment and electroplating processes used for continuous plating of strip, wire, and other products. The invention, therefore, covers the application of the principle in other processes such as acid zinc, acid copper, cyanide zinc, cyanide copper, nickel, and the like.

The major advantage of operating with a conducting shield is to avoid plating on the rolls, minimize need for sanding and thereby prolong period of operation before developing a bipolar plating condition. The shield may not prevent plating on rolls which have reached this stage of deterioration because the electrical characteristics of the system are quite different.

What is claimed is:

1. In a process for the continuous electrotinning of a wire or strip substrate from an electroplating bath to provide a tin coating thereon, the improvement comprising: immersing a metallic shield into the bath near and essentially parallel to the axis of the cathode contact roll and between the anode and cathode contact roll and maintaining the shield at a potential about 0.03 to 0.2 volt more negative than the surface of the cathode contact roll.

2. The process of claim 1 wherein the electroplating bath is a halogen tin plating bath.

3. The process of claim 2 wherein the metallic shield has a contour substantially the contour of the roll.

References Cited UNITED STATES PATENTS 485,343 11/1892 Fletcher 204-12 2,044,415 6/1936 Yates 20413 2,569,577 10/1951 Reading 204-28 3,023,154 2/1962 Hough et al. 204242 FREDERICK C. EDMUN-DSON, Primary Examiner US. Cl. X.R. 204Dig. 7, 54 

